Electrochemical machining electrolyte

ABSTRACT

Hygroscopic additives for sodium and potassium chlorate and perchlorate electrochemical machining electrolytes. These additives have a fire-retardant effect on combustibles exposed to electrolyte splash. When used in conjunction with relative humidities of above about 35 percent, the additives prevent the electrolyte splash from drying sufficiently to support combustion. Specific materials which are compatible with the electrolyte are disclosed.

Ilnit States Patent [72] Inventors Jacob B. Darling Carmel, Ind; Mitchell A. LaBoda, Detroit, Mich. [21] AppLNo. 750,772 [22] Filed Aug.7, 1968 [45] Patented Oct. 26, 1971 [73] Assignee General Motors Corporation Detroit, Mich.

[54] ELECTROCHEMICAL MACHINING IEILIECTROLYTE 3 Claims, No Drawings [52] U.S. Cl 204/143, 252/2 [51] lint. Cl 823p 1/00 [50] lField of Search 204/1435, 143.6;252/8.1, 194

[56] References Cited UNITED STATES PATENTS 2,805,197 9/1957 Thibault et a1 204/ 1 43 X ABSTRACT: Hygroscopic additives for sodium and potassium chlorate and perchlorate electrochemical machining electrolytes. These additives have a fire-retardant efiect on combustibles exposed to electrolyte splash. When used in conjunction with relative humidities of above about 35 percent, the additives prevent the electrolyte splash from drying suiticiently to support combustion. Specific materials which are compatible with the electrolyte are disclosed.

ELECTROCHEMICAL MACHINING ELECTROLYTIE This invention relates generally to electrochemical machining electrolytes and more specifically toward alkali metal chlorate and/or perchlorate-based electrolytes and fire-retardant additives therefor.

This invention is an improvement to the electrolyte disclosed and claimed in copending U.S. Pat. application, Ser. No. 664,770 now abandoned entitled Electrochemical Machining Electrolyte filed on Aug. 31, 1967, in the name of Mitchell A. LaBoda and assigned to the assignee of the incident invention.

It is intended that the aforesaid patent application be incorporated herein by reference. The aforesaid copending application relates to alkali metal chlorate and perchlorate-based electrolytes. These electrolytes, hereafter referred to simply as chlorate electrolytes," have revolutionized the electrochemical machining industry by providing a unique combination of properties which permit the rapid and accurate machining of metals with little or no overcut or splash-cutting while at the same time producing machined surfaces having finishes as low as 2-5 microinches.

Although the use of these chlorate and perchlorate-based electrolytes have recently taken on considerable commercial importance in the electrochemical machining industry, it is not without concern over a particular disadvantage associated with their use. This disadvantage is the degree of care with which these electrolytes should be handled and used. When in solution, the chlorate and perchlorate salts do not present a problem, per so. However, when dry and in contact with organic combustible materials, such as paper, cloth or wood, the possibility of combustion arises. As a result, the electrolyte has heretofore been used under closely controlled conditions. Special clothing is often worn in the work area. Special handling of the clothing used in the work area was necessitated. Other precautionary measures have also been taken. While the precautions taken in conjunction with the use of chlorate and/or perchlorate electrolytes are no more stringent than for many other industrially used chemicals, it would, nonetheless, be desirable to eliminate, as much as possible, the need for such precautions. The elimination or substantial reduction of the need for such precaution contributes to the economic benefits of the use of the chlorate and/or perchlorate electrolytes.

Preferably it is desirable to merely add something to the electrolyte which would reduce the possibility of fire. However, the chlorate and perchlorate electrolytes are somewhat sensitive to the addition of high concentrations of many foreign ions, particularly anions. For example, more than about a 5 percent addition of chloride ion-bearing compounds have a detrimental affect on the low-throwing characteristic of the chlorate electrolytes so that wild cutting becomes increasingly apparent and dimensional control is more difficult to hold. On the other hand, a 5 percent chloride ion addition does not appreciably affect the surface finish or cutting rate characteristics of the chlorate electrolytes. Other foreign materials tend to affect one or more of the three principal characteristic properties of the chlorate electrolyte.

We have now found that certain materials can be added to chlorate electrolytes to reduce the likelihood of fire and which materials are compatible with the electrochemical machining characteristics of these electrolytes.

It is, therefore, an object of this invention to reduce the likelihood of combustion which might otherwise occur incident to the inadvertent or accidental splashing and subsequent drying of chlorate electrolytes on combustibles and to do so by adding to the electrolyte hygroscopic compounds which are substantially compatible with the unique electrochemical machining characteristics of chlorate electrolytes.

This and other objects and benefits of the incident invention will become more apparent from the detailed disclosure which follows.

Briefly stated, this invention relates to the addition of ECM- compatible hygroscopic materials to chlorate electrochemical machining electrolytes. By ECM-compatible materials are meant materials which when added to chlorate ECM electrolytes do not materially interfere with any of the three principal ECM characteristics for which the chlorate electrolytes are unique. When used in conjunction with relative humidities controlled to be in excess of about 35 percent, the hygroscopic materials will retain or absorb sufficient moisture from the ambient air to preclude the "splash" from drying to a degree sufficient for combustion. When the relative humidity is in excess of about 72 percent, the chlorate and perchlorate salts are sufficiently hygroscopic themselves to prevent drying. For relative humidities below about 72 percent, but in excess of about 59 percent, lithium bromide, potassium iodide, magnesium perchlorate, lithium citrate, potassium lactate and potassium acetate may be used in concentrations of less than about 50 g./l. When the relative humidity is as low as about 35 percent, lithium bromide and potassium acetate have been found to be most effective.

As disclosed and claimed in the U.S. Pat. application, Ser. No. 664,770, chlorate electrochemical machining electrolytes have effectively been used at concentrations of about $0 g./l. of the salt upwards to about the saturation point of the salt in the solution. The most practical operating concentration range of the chlorate appears to be about 350-500 g./l. As indicated heretofore, under certain conditions dried chlorate electrolyte in contact with combustible organic matter increases the likelihood or risk of fire. In this regard, generally it is noted that the higher the concentration of the chlorate salt in the electrolyte, the less chance there is fire. This is so because the chlorate salt is hygroscopic and will itself absorb sufficient moisture from the atmosphere to prevent ignition and/or retard burning rates. This is true even at moderate relative humidities. It has also been noted that at relative humidities in excess of about 70-72 percent, even the lower concentration chlorate solutions are not a significant fire hazard, because the electrolyte splash will not dry sufficiently to cause a fire. By this invention this fire immunity is extended to chlorate electrolytes having low concentrations of chlorates (e.g., less than about 500 g./l.) and which are used in environments having relative humidities less than about 72 percent. lt has been found chlorate electrolytes without being materially detrimental to the principal ECM characteristics of the chlorate electrolytes. These materials give the chlorate electrolytes the desired fire immunity. lt is noted that no salts have been found which are perfectly ECM-compatible with the chlorate electrolytes. Many have been found which are imcompatible for one reason or another. Hence the truly effective additives involve a compromise of benefits. in this regard, for example, we have noted generally that salts such as lithium bromides and potassium iodide have virtually no affect on the cutting rate or the surface finish benefits obtainable from the chlorate electrolytes. However, when used in concentrations above about 50 g./l., lithium bromide and lithium iodide tend to affect the accuracy with which a piece may be machined by reducing somewhat the low-throwing power characteristic of the electrolyte which in turn gives rise to some uncontrolled or wild cutting. When these salts are used in concentrations below about 50 g./l., the affects on the low-throwing power characteristic might well be tolerated in view of the fire immunity benefits obtainable by their addition. Other additives have other affects. For example, salts such as lithium citrate, potassium lactate and potassium acetate have been found to slow down the cutting rate of the chlorate electrolytes but seen to have no affect on the surface finish and/or wild-cutting characteristics of that electrolyte. Generally speaking, it appears that the inorganic salts tend to affect only the lowthrowing power characteristic, and the organic salts tend to affect only the cutting rate characteristic of chlorate electrolytes.

A particularly interesting compound for purposes of this invention is magnesium perchlorate if used in concentrations of about 37 g./l. or more, and in an environment of at least about 60 percent relative humidity (i.e., about 60 percent or more).

It is very effective as a fire-retardant and additionally proves the benefit of being the most nearly ECM-compatible of all the additives tested, apparently owing to the commonality of the anions involved. Magnesium perchlorate is not recommended for use in environments wherein the relative humidity is appreciably lower than the aforementioned 60 percent relative humidity. This is so because it has been noted that at lower relative humidities the magnesium perchlorate tends to accelerate the burning rate rather than decrease the likelihood of fire.

As shown in Table i, which follows, each additive has its own concentration range requirements for providing acceptable results, With the exception of magnesium perchlorate, the acceptable concentration range appears to be between about 5 g./l. to about 50 g./l. At concentrations below the minimum value little or no fire retardant benefits are observed. Generally at concentrations above about 50 g./l., the ECM- compatibility deteriorated rapidly to the point where many of the chlorate electrolyte advantages are lost. Magnesium perchlorate, on the other hand, should have at least about 37 g./l. for effective fire-retardancy and has no practical discernible upper limit owing to compatibility of the anions. On the other hand, because of the 60 percent relative humidity requirement, magnesium perchlorate lacks the quality of being universally useful over a broad range of humidities.

Table l reflects some preferred additives and the concentrations found to be useful in accordance with this invention.

Table ll reflects the results of a series of tests on paper to determine the fire-retardancy benefits of certain salts within our invention when used at relative humidities of about 59-63 percent.

' TABLE 11 2#/gel. chlorate 3#/gnl. chlorate Burning Burning Impact rate, Impact rate, sensitive in./sec. sensitive in./sec.

Untreated No 1. 14 No 1. 14 NaClO; (only) Yes 2. 11 Yes 1.91 LtBr, g./l.:

Yes 0. 71 Yes 1. 09-1. 15 N o 0. 12 No 0 No 0 No 0 No 0 No 0 N0 1. 140 Yes 0. 75 N0 0. 16-0 Z 0 1. 06 N0 0. 16-0 No 0. 14-0 N0 0 N0 0. 09-0 No 0 No 0 No 0 N0 0 No 0 No 0 No 0 No 0 No 0 No 0. 80-0 No 0 No 0 N0 0 No 0 No 0 No 0 Yes 1. 20 Yes 0. 86 Yes 1. 50 Yes 1. 38 Yes 1. 50 Yes 1. 24 Yes 0. 84 Yes 0. 80

No 0 Yes 1. 20 No 0 Yes 1. 34 No 0 Yes 1. 09 No 0 Table III reflects the results of a series of tests on paper to determine the fire-retardancy benefits of certain salts within our invention when used at relative humidities of about 35-38 percent.

TABLE I11 2#/ga1. NeClO; 3#/ge1. C103 Burning Burning Impact rate, Impact rate,

sensitive in./see. sensitive in./sec.

Untreated No 1. 14 No 1. 14

NaCiOa (only) Yes Yes 2. 27 LiBr, g./1.:

Yes 2. 12 Yes 1. 11

Yes 0. 95 Yes 1. 20

Yes 0. 51 Yes 1. 38

Yes 1. 29 Yes 1. 17

Yes 1. 29 Yes 1. 33

Yes 0. 79 Yes 1. 56

Yes 0. 46 Yes 0. 86

Table IV reflects the results of a series of tests on shop cloths to determine the fire-retardancy benefits of certain salts within our invention when used at relative humidities of about 59-63 percent.

TABLE IV 2#/gnl. NaCiO; 3#/gel. C10:

Impact rate, Impact rate,

sensitive in./sec. sensitive in./sec.

Untreated No 0. 28 No 0. 28

NeOlOa (only) Yes 2.27 Yes 1. 29 LiBr, g./l.:

No 0 No 0 No 0 No 0 No 0 N o 0 N0 0 No 0 No 0 N 0 0 86-0 No 0 No 0 No 0 No 0 No 0 No 0 Yes 1. 20 No 0 Yes 1. 00 No 0 No 0. 51 No 0 No 0 N 0 0 No 0 N0 0 No 0 No 0 No 0 No 0 Yes 1. 17 Yes 1. 11

Yes 0. 57 Yes 0. 50

Yes 0. 37 Yes 0. 55

Yes 0. 09 Yes 0. 08

N0 0 Yes 1. 17

No 0 N0 0. 46

No 0 No 0 Table V reflects the results of a series of tests on shop cloths to determine fire-retardancy benefits of certain salts within our invention when used at relative humidities of about 35-38 percent.

TABLE V 2#lgal. NnClOa 3#/ga1. NeClO;

Burning Burning Impact rate, Impact rate,

sensitive in./sec. sensitive in./sec.

Untreated N o 0. 28 N0 0. 28

NaCiO Yes 2. 27 Yes 2. 56 LiBr, g./1.:

Yes 1. 50 Yes 1. 79

Yes 1. 12 Yes 1. 79

Yes 0. 59 Yes 1. 79

N o 0. 09 N0 0. 01

Yes 1. 09 Yes 1. 31

Yes 0. 40 Yes 0. 90

Yes 0. 41 Yes 0. 66

N o 0. 08 Yes 0. 24

Table VI reflects the ECM-compatibility of certain salts within our invention.

the box was controlled by use of combinations of salts in contact with their saturated solutions as described in the Hand- TABLE VI 2#/gal. N 9.0103 3#/gal. N 21010 12 volts 16 volts 12 volts 16 volts A B C A B C A B C A B C Standard sample 0. 012 E E 117 E E 0. 102 E E 0. 128 E E E E 0. 103 E E 0. 097 E E 0. 124 E E E E 0. 105 E E 0. 100 E E 0. 136 E E G E 0. 114 G E O. 108 G E 0. 125 G E G E 0. 116 G E 0. 108 G E 0. 124 G E E E 0. 123 G E 0. 113 E E 0. 136 G E G E O. 124 G E 0. 106 G E 0. 138 G E G E 0. 130 G E 0. 110 G E 0. 137 G E G E 0. 125 G E 0. 114 G E 0. 141 G E E E 0. 095 E E 0. 092 E E 0. 123 E E E E 0. 092 E E 0. 101 G E 0. 136 E E E E 0. 088 E E 0. 092 E E 0. 099 E E G E 0. 068 E E 0. 076 G E 0. 089 E E G E 0. 068 E E E E 0. 086 E E 0. 090 E E 0. 103 E E E E 0. 088 E E 0. 077 E E 0. 115 E E E E 0. 082 E E 0. 077 E E 0. 100 E E E E 0. 082 E E 0. 064 G E 0. 096 G E E E 0. 111 E E 0. 096 E E 0. 122 E E E E 0. 112 E E 0. 092 E E 0. 111 E E E E 0. 109 E E 0. 077 E E 0. 107 E E E E 0. 083 E E Legend: A=metal removal rate in./min.; B=lack Ratings: E=excellent; G=good; F=fair; P=poor.

For purposes of these tests, the preferred standard of acceptability for a fire-retardant additive was whether that additive would produce a burning rate which was approximately equal to or less than the burning rate of the untreated test sample while at the same time be compatible with the ECM characteristics of the chlorate electrolytes. A less rigid, but nonetheless acceptable, standard is whether the particular additive would reduce the burning rate of the test sample below that of samples treated only with NaClO electrolytes without additives.

The data reported in the tables was derived from experiments briefly described hereafter. Two commonly found organic materials (i.e., paper toweling and shop cloths) were selected as the test materials. Untreated towels and shop cloths were used as the first standard. Towels and shop cloths treated in electrolytes which did not have the hygroscopic additives were used as the second standard. All other towels and shop cloths were treated in various solutions of electrolytes containing different additives at varying concentration levels.

Stock solutions of sodium chlorate containing about 2 lbs/gal. 230 g./l.) and about 3 lbs/gal. (350 g./l.), were used for all experiments. Additions of the additives to be investigated were made to l-liter portions of the stock solutions. All samples to be tested were equilibrated at the same relative humidity range being investigated. 2"Xl4" strips of treated materials were cut from paper toweling and the shop cloth. The paper toweling was identified as Scott C-Fold, Brand 150, ultra-high absorbency white. The shop Cloth selected was identified as KEX National Service. The strips were soaked in the electrolytes to be tested and were hung out to drip dry for about 4 hours in the laboratory which had a relative humidity of about 40 percent. After this drip-dry period, the strips were equilibrated in a controlled humidity box for a period of at least 24 hours. The controlled humidity box was a polyethylene tank l8" 18"X24" fitted with a sealing lid. The tank was made airtight by means of sealing around the edges with tape. Air movement within the box was effected by a motor-propeller arrangement. A relative humidity recorder was used inside the chamber to monitor both the relative humidity and the temperature within an accuracy of about :2 percent relative humidity and about :1" F. The humidity in 01 wild cutting; C=surtace finish.

book of Chemistry and Physics, p. 21,309, Chemical Rubber Publishing Co., Cleveland, Ohio (1953-l954). The salt used in the 59-63 percent relative humidity range and in the 72 percent relative humidity experiments was sodium bisulfate. In the 35-38 percent relative humidity range chromic acid anhydride was used in the controlled humidity box. The burning rate tests were conducted in a steel box made to hold a sample rack in a vertical position. A Bunson Burner flame, 1% inches high was used to ignite the samples; ignition time was 1 second for the paper samples, and 5 seconds for the cloth samples. 12- inch portions of the samples were ignited in the burning chamber. Burning time was measured and a burning rate calculated.

In addition to the burning rate tests, impact tests were also conducted. For the impact sensitivity test, 2-inch squares were cut from the ends of the paper and cloth strips tested in the burning rack. The squares were placed on a tile and manually struck several times with a ball peen hammer swung through an arc of about 12 inches.

As indicated before, the preferred materials which are considered to be most useful are those which had burning rates equal to or less than the ordinary, untreated samples and would produce these low rates at concentrations low enough so as not to materially interfere with the ECM characteristics defined above. In this regard, for example, it was found that several materials were efiective for fire retardancy, but required higher concentrations than thought to be compatible with the ECM characteristics of the chlorate solution.

The ECM-compatibility tests were conducted using a 5160- H steel tube ground electrochemically across its face for 1 minute using a gravity-feed system. The tube was ground using a Copperdyne aluminum oxide wheel made by Bay State Abrasive Products Company of West Borough, Massachusetts. Tests were conducted using both 12 and 16 volts. The tube lengths were measured with a micrometer before and after electrochemical machining to determine the metal removal rate. The tube ends were next subjectively compared under a binocular microscope at 20 magnifications. The basis for this subjective comparison was twofold, the first being a change in the reflectivity of the surface, and the other being a change in the tube-end profile, the latter being an indication of any wild-cutting activity.

Breakdown of some of the hygroscopic additive materials was observed in a number of cases when the electrolyte was subjected to electrochemical machining conditions. Accordingly, periodic additions of the hygroscopic agent may be necessary to hold the concentration of the additive at an effective level.

While our invention has been described solely in terms of certain specific embodiments thereof, we do not intend to be limited thereto but rather only to the extent defined hereafter.

We claim:

1. A method for reducing the possibility of fires in work areas surrounding electrochemical machining apparatus using an aqueous solution of at least one salt selected from the group consisting of sodium chlorate, potassium chlorate, sodium perchlorate and potassium perchlorate, said method comprising the steps of adding to said solution at least one ECM- compatible, ignition-suppressant, which ignition-suppressant, when out of solution, is sufiiciently hygroscopic at relative humidities in excess of about 35 percent and less than about 72 percent to preclude mixtures of said salt and said suppressant from drying sufficiently to permit ignition of combustible materials in contact with said mixtures, and maintaining a relative humidity of at least about 35 percent in said work area.

2. A method for reducing the possibility of tire in work areas surrounding electrochemical machining apparatus using an aqueous solution of sodium or potassium chlorate, said method comprising the steps of adding to said solution at least one hygroscopic, ignition-suppressant selected from the group consisting of at least about 5 g./l. and less than about 25 g./l. lithium bromide, at least about 15 g./l. and less than 25 g./l. potassium iodide, at least about 37 g./l. potassium perchlorate, at least about 5 g./l. and less than about 15 g./l. lithium citrate, at least about 5 g./l. and less than about 50 g./l. potassium acetate, and maintaining a relative humidity of at least about 59 percent in said work area.

3. A method for reducing the possibility of fire in work areas surrounding electrochemical machining apparatus using an aqueous solution of sodium or potassium chlorate, said method comprising the steps of adding to said solutions at least about 25 g./l. and less than about 50 g./l. of a hygroscopic, ignition-suppressant selected from the group consisting of lithium bromide and potassium acetate and maintaining a relative humidity of at least about 35 percent in said work area.

W UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,616,350 Dated October 26, 1971 Jacob B. Darling and Mitchell A. LaBoda lnvencor (is) It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

In the specification: Column 5, Table VI, under the column headed 2#/gal. NaClO3 -l2 volts-A delete "0.012" and substitute -O.lO2--.

In the claims: Column 8, line 9 delete "potassium" second occurrence and substitute --magnesiumtherefor: Column 8, line 11 after "potassium" insert -lactate, and at least about 25 g/l and less than about 50 g/l potassium.

Signed and sealed this 23rd day of January 1973.

(SEAL) Attest:

ROBERT GOTTSCHALK EDWARD M. FLETCHER ,JR

Commissioner of Patents Attesting Officer 

2. A method for reducing the possibility of fire in work areas surrounding electrochemical machining apparatus using an aqueous solution of sodium or potassium chlorate, said method comprising the steps of adding to said solution at least one hygroscopic, ignition-suppressant selected from the group consisting of at least about 5 g./l. and less than about 25 g./l. lithium bromide, at least about 15 g./l. and less than 25 g./l. potassium iodide, at least about 37 g./l. potassium perchlorate, at least about 5 g./l. and less than about 15 g./l. lithium citrate, at least about 5 g./l. and less than about 50 g./l. potassium lactate, and at least about 25 g./l. and less than about 50 g./l. potassium acetate and maintaining a relative humidity of at least about 59 percent in said work area.
 3. A method for reducing the possibility of fire in work areas surrounding electrochemical machining apparatus using an aqueous solution of sodium or potassium chlorate, said method comprising the steps of adding to said solutions at least about 25 g./l. and less than about 50 g./l. of a hygroscopic, ignition-suppressant selected from the group consisting of lithium bromide and potassium acetate and maintaining a relative humidity of at least about 35 percent in said work area. 